Data rates continue to increase in digital systems, communication systems, computer systems, and in other applications. In such applications, various devices communicate data using signals that are encoded with information in the form of signal levels (e.g., amplitude) in certain intervals of time. Proper decoding of periodic signals, for example, involves measuring the signal level in the correct time interval, or period. As data rates increase, margins of error in the signal level timing tend to decrease.
In general, errors in which signal event deviates in its timing from an ideal timing is sometimes referred to as “jitter.”
Jitter may be introduced to a signal from a variety of sources. For example, jitter may be added to a signal by circuit elements used to generate, convey, and/or receive the signal. To various degrees, circuit elements may add jitter to the signal through cross-talk, reflections, shot noise, flicker noise, and/or thermal noise. Electromagnetic interference (EMI) may also contribute to jitter.
Typically, jitter is measured as a total jitter. Total jitter may represent a convolution of all independent jitter components, which include contributions from deterministic and random components. Random jitter, such as that caused by noise, typically exhibits a Gaussian distribution. Deterministic jitter may include periodic jitter, duty cycle distortion, and intersymbol interference.
Jitter measurements may be made in a variety of applications, examples of which may include Fibre Channel, Gigabit Ethernet, XAUI, InfiniBand, SONET, Serial ATA, 3GIO, and Firewire. To illustrate the importance of jitter in such applications, a nanosecond of jitter in a 100baseT (100 Mb/s) device may represent a 10% data uncertainty. For example, the same nanosecond of jitter may represent a 20% data uncertainty if the data rate is increased to 200 Mb/s, or a 100% data uncertainty in a Gigabit Ethernet device.